Wiper timing and geometry to minimize sensor occlusion

ABSTRACT

This technology relates to a system for clearing a sensor cover. The system may be comprised of a first sensor that rotates within a sensor cover, a plurality of second sensors that are fixed relative to the sensor cover, a first wiper that is configured to clear the sensor cover of debris, and a motor. The motor may rotate the first wiper in a first direction at a first predetermined rotation rate defined at least in part by a second predetermined rotation rate of the first sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 16/781,126, filed Feb. 4, 2020, which is a continuation of U.S.patent application Ser. No. 15/784,944, filed Oct. 16, 2017, now U.S.Pat. No. 10,589,726, issued Mar. 17, 2020, the entire disclosures ofwhich are each incorporated herein by reference.

BACKGROUND

Various types of vehicles, such as cars, trucks, motorcycles, busses,boats, airplanes, helicopters, lawn mowers, recreational vehicles,amusement park vehicles, farm equipment, construction equipment, trams,golf carts, trains, trolleys, etc., may be equipped with various typesof sensors in order to detect objects in the vehicle's environment. Forexample, vehicles, such as autonomous vehicles, may include such LIDAR,radar, sonar, camera, or other such imaging sensors that scan and recorddata from the vehicle's environment. Sensor data from one or more ofthese sensors may be used to detect objects and their respectivecharacteristics (position, shape, heading, speed, etc.).

However, these vehicles are often subjected to environmental elementssuch as rain, snow, dirt, etc., which can cause a buildup of debris andcontaminants on these sensors. Typically, the sensors include a cover toprotect the internal sensor components of the sensors from the debrisand contaminants, but over time, the cover itself may become dirty. Assuch, the functions of the sensor components may be impeded as signalstransmitted and received by the internal sensor components are blockedby the debris and contaminants.

SUMMARY

Embodiments within the disclosure generally relate to clearing vehiclesensors of debris and contaminants. One aspect includes a system forclearing a sensor cover. The system comprising a first wiper and secondwiper configured to clear the sensor cover of debris and a motor forrotating the first wiper and the second wiper in a first direction at afirst predetermined rotation rate defined at least in part by a secondpredetermined rotation rate of a first sensor arranged within the sensorcover. The first wiper and the second wiper may be offset by apredetermined angular distance relative to a center of the sensor cover.

In some instances the first predetermined rotation rate is furtherdefined by a total number of a plurality of second sensors, the secondsensors being fixed relative to the sensor cover. The predeterminedangular distance apart may be between 150 and 170 degrees when the totalnumber is 8.

In some instances the system comprises the plurality of second sensors.The plurality of second sensors may be cameras.

In some instances system may further comprise the first sensor, andwherein first sensor may be configured to rotate in a second directionat the second predetermined rotation rate.

In some instances the first predetermined rotation rate is furtherdefined by the number of spaces between the plurality of second sensorsthe first or second wiper will advance per cycle where a cycle is thedifference between the first or second wiper position and a position ofthe first sensor repeats with some periodicity. In some instances thefirst predetermined rotation rate is further defined by dividing thenumber of spaces between the plurality of second sensors by a number ofspaces between the plurality of second sensors the first sensor willadvance per cycle. In some instances the first predetermined rotationrate is the result of the dividing, multiplied by the secondpredetermined rotation rate of the first sensor.

In some instances the first predetermined rotation rate is defined bywiper_advances_per_cycle/sensor_advances_per_cycle*f_sensor_rotation,where wiper_advances_per_cycle is the number of spaces between thecameras the first or second wiper will advance per cycle,f_sensor_rotation is the second predefined rotation rate, andsensor_advances_per_cycle is the number of spaces between the pluralityof second sensors the first sensor will advance per cycle where a cycleis the difference between the first or second wiper position and aposition of the first sensor repeats with some periodicity. In someinstances sensor_advances_per_cycle is defined in part by the number ofsecondary sensors multiplied by the number of rotations the first sensorrotates per cycle. In some instances sensor_advances_per_cycle isfurther defined in part by the wiper_advances_per_cycle multiplied by awiper direction value, where the wiper direction value is 1 when thefirst and second wipers rotate in the same direction as the first sensorand −1 when the first and second wipers rotate in the opposite directionas the first sensor.

In some instances sensor_advances_per_cycle is defined bysensor_rotations_per_cycle*num_sensors+wiper_direction*wiper_advances_per_cycle,where num_sensors is the number of secondary sensors, wiper_direction is1 when the first and second wipers rotate in the same direction as thefirst sensor and −1 when the first and second wipers rotate in theopposite direction as the first sensor, and sensor_rotations_per_cycleis the number of rotations the first sensor rotates per cycle.

In some instances the predetermined angular distance is defined by thetotal number of a plurality of second sensors and which second sensorsthe wipers should be between during the first rotation of a firstsensor.

In some instances the predetermined angular distance is further definedby the rotation rate of the first sensor and a rotation rate anddirection of the first and second wipers.

In some instances the predetermined angular distance is further definedby((slot_numbers+wiper_direction/2)/num_sensors*2*Å*(1.0−wiper_direction*wiper_frequency/f_sensor_rotation)),where slot_numbers is an index of the position between the secondarysensors that a given wiper should be in during the first rotation of thefirst sensor, num_sensors is the number of secondary sensors,wiper_direction is 1 when the first and second wipers rotate in the samedirection as the first sensor and −1 when the first and second wipersrotate in the opposite direction as the first sensor, f_sensor_rotationis the rotation rate of the first sensor, and wiper_frequency is thefirst predetermined rotation rate.

In some instances the system further comprises a third wiper. The firstwiper, the second wiper, and the third wiper may be offset by apredetermined angular distance relative to a center of the sensor cover.

In some instances the first and second wipers each include one or morewiper blades. In some instances the one or more wiper blades areconfigured to clear the sensor cover by wiping the sensor cover wiper inthe first direction at the first predetermined rotation rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The present technology is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which likereference numerals refer to similar elements including:

FIGS. 1A and 1B are illustrations of a sensor and wiper configuration inaccordance with aspects of the disclosure.

FIG. 2 shows a sensor cover window in accordance with aspects of thedisclosure.

FIG. 3 shows a top down view of imaging sensors within a sensor inaccordance with aspects of the disclosure.

FIG. 4 is an illustration of field of views for cameras within thesensor in accordance the disclosure.

FIG. 5 is an illustration of a camera capturing a frame in accordancewith aspects of the disclosure.

FIG. 6 is an illustration of a wiper being rotated around a sensor inaccordance with aspects of the disclosure.

FIG. 7A-7D are illustrations of the operation a sensor with a singlewiper being engaged in accordance with aspects of the disclosure.

FIGS. 8A and 8B are illustrations of two wiper positioned opposite ofeach other being rotated around a sensor in accordance with aspects ofthe disclosure.

FIGS. 9A-9D are illustrations of the operation of a sensor with twowipers positioned opposite of each other being engaged in accordancewith aspects of the disclosure.

FIGS. 10A-10D are illustrations of the operation of a sensor with twowipers offset from each other being engaged in accordance with aspectsof the disclosure.

FIG. 11 shows a plot of two offset wipers on a sensor cover over aperiod of time relative to a position of the sensor cover at whichcameras are imaging.

FIG. 12 shows a plot of three offset wipers on a sensor cover over aperiod of time relative to a position of the sensor cover at whichcameras are imaging

DETAILED DESCRIPTION

The technology described herein relates to clearing vehicle sensors ofdebris and contaminants to assure adequate operation. For instance, asensor may include a cover to protect internal sensor components fromdebris such as water, dirt, insects, and other contaminants, but, asnoted above, the cover itself may become dirty over time. As such, thefunctions of the internal sensor components may be impeded as signalstransmitted and received by the internal sensor components may beblocked by the debris. Debris may be cleared from a sensor by rotatingtwo or more wipers around the sensor offset from one another by apredetermined angular distance, such that the cover is wiped clear bythe wipers.

A vehicle sensor may be comprised of internal sensor components, a coverfor housing the internal sensor components, and a cover window. Thecover window may be constructed at a specific location on the sensorcover and the internal sensor components may transmit and receive one ormore signals through the cover window.

The internal sensor components may include one or more imaging sensors,such as rotatable sensors or stationary sensors configured to capturedata corresponding to the sensor's surroundings. For instance, arotatable sensor may be configured to continually capture data whilestationary sensors, such as cameras, may be configured to periodicallycapture image data.

The rotation rate of the wipers around the sensor may be set such thatthe wipers are positioned outside the field of view of the stationarysensors as they capture an image. In this regard, when only a singlewiper is used, the wiper may be rotated at such as speed to locate thewiper approximately halfway between the apertures of neighboringcameras, in a part of the dome with no aperture, when the camera isimaging. However, a single wiper may not be able to clear the coverquickly enough and the use of more than one wiper spaced uniformlyopposite each other may result in one of the wipers entering the fieldof view of a sensor as it transmits and receives data. More rapidclearing of the sensor cover may be needed than a single wiper canhandle. As such, more than one wiper may be used. In this regard,additional wipers may be used in conjunction with the first wiper, andall of the wipers may be rotated around the sensor at the same rate.

The wipers should be positioned such that they are offset from eachother at a predetermined angular distance relative to the center of thesensor cover. In this regard, if two wipers are positioned opposite eachother relative to the center of the sensor cover, the wipers may bepositioned within the field of view of a stationary sensor when thestationary sensor is capturing an image. By offsetting the wipers at apredetermined angular distance, the wipers may be consistentlypositioned between apertures of neighboring stationary sensors, therebyavoiding blocking a stationary sensor's field of view during itsexposure duration.

The features described herein may allow for continued use of a sensoreven when the sensor's cover becomes dirty. By doing such, the sensormay continue operation without interruption or the need for anindividual to manually clean the sensor, as the wiper may continuallyand rapidly clean the sensor cover when needed. As such, the vehicle maycontinually operate in environments which produce a lot of debris, suchas construction sites or off-road locations for road vehicles such ascars, trucks, etc. However, while certain aspects of the disclosure areparticularly useful in connection with specific types of vehicles, thevehicle may be any type of vehicle including, but not limited to, cars,trucks, motorcycles, busses, boats, airplanes, helicopters, lawn mowers,recreational vehicles, amusement park vehicles, farm equipment,construction equipment, trams, golf carts, trains, and trolleys.

A vehicle sensor may be comprised of internal sensor components and acover for housing the sensor components. For instance, FIGS. 1A and 1Bare side and top-down views of a sensor 100 having a sensor cover 115and wipers 120 and 125. As noted above, the wipers 120 and 125 may beoffset from each other by a predetermined angular distance. Each of thewipers may be rotated in a first direction 130 around the cover 115 ofthe sensor 100. As the wipers rotate around the sensor 100, the wiperblades 121 and 126, of wipers 120 and 125, respectively, may loosen,pull and push away the debris built up on the cover.

The cover of the sensor may be configured in various shapes and sizes.For instance, as shown in FIG. 2 , the sensor cover 115 may beconfigured such that it has a domed shaped top portion 210 with a sidewall 205 in the shape of a frustum. The sensor cover 115 may becomprised of materials such as plastic, glass, polycarbonate,polystyrene, acrylic, polyester, etc. The sensor may be locatedinternally or externally from a vehicle.

The cover of the sensor may include a cover window through which theinternal sensor components may transmit and receive signals. Forinstance, as further shown in FIG. 2 , the entire side wall 205 of thesensor cover 115 may be constructed as a cover window 215, to allowsignals 250 to penetrate the sensor cover 115. Although the entire sidewall 205 is shown as being the cover window 215, in some instances onlya portion or portions of the sidewall may be constructed as coverwindows. The cover window 215 may be composed of the same, or different,material as the sensor cover 115. In some instances the entire sensorcover 115, or a large portion of the sensor cover, may be penetrable bythe signals 250 transmitted and received by the internal sensorcomponents, thereby allowing the entire sensor cover 115 to function asa cover window.

The internal sensor components may transmit and receive one or moresignals through the cover window of the sensor. In this regard, theinternal sensor components may include one or more imaging sensors suchas LIDAR, radar, sonar, camera, or other such imaging sensors positionedwithin the cover of the sensor. For instance, as shown in the top downview of a sensor in FIG. 3 , the internal sensor components may includeeight stationary sensors, such as cameras 300-314 in a ring formation,spaced 45 degrees apart in the azimuth, and a rotatable sensor, such asa LIDAR 320, positioned in the center of the sensor 100. The LIDAR 320may be positioned above or below the cameras 300-314. Although a singleLIDAR 320 and eight cameras 300-314 are shown as being within the sensor100, any number and combination of sensors may be used.

The rotatable sensor may be configured to continually capture portionsof the sensor's surroundings. In this regard, the rotatable sensor maybe configured to rotate at a predetermined rate and capture a range ofthe sensor's surroundings. For instance, referring to the example ofFIG. 3 , the LIDAR 320 may be configured to rotate a speed of 10 Hz, ormore or less, in the first direction 321. As the LIDAR 320 rotates, theLIDAR may continually transmit and receive signals 322 to capture imagedata corresponding to a degree range X, of its surroundings. This degreerange X may be about 10 degrees, or more or less.

The cameras may be configured to capture image frames which include datacorresponding to the sensor's surroundings. For instance, as shown inFIG. 4 , each camera 300-314 may have a field of view 400-414,respectively, within which it captures image frames. In order to capturea full image frame, each camera may require 3 cm, or more or less, ofunimpeded space on the cover window corresponding to the respectivecamera's field of view.

The angle and orientation of each camera may be positioned to eliminateblind spots. In this regard, the positioning of the cameras may be suchthat their respective fields of views form a cohesive, 360-degree fieldof view of the vehicle's surroundings when combined. In certaininstances, the cameras may be calibrated and tested to confirm that thecameras are performing as expected and/or to provide data, such as imagedata, which can be analyzed to improve the design and features of thecameras. For instance, the cameras 300-314 may be calibrated and testedto assure they are providing a full 360 degree field of view of thevehicle's surroundings.

Each camera may be configured to capture a frame of data upon therotatable sensor aligning with the field of view of the respectivecamera. Continuing the example of the LIDAR 320 rotating at 10 Hz, eachcamera's field of view may align with the LIDAR every 100 ms. As such,each camera may capture a frame every 100 ms. For instance, as shown inFIG. 5 , upon the LIDAR 320 aligning with camera 308, such as with thecenter of the camera's field of view, the camera 308 may capture a frameof image data within its field of view 408. In some instances, eachcamera will have exposure duration plus read time that is a smallduration of the time between frames (i.e., less than 100 ms). Thus, eachcamera may be inactive (i.e., not capturing an image) for a period oftime, such as more than half the duration of the time between frames.

The one or more wipers may be comprised of a wiper blade, a wipersupport, and a wiper arm. For instance, referring to FIG. 6 , a singlewiper 601 is shown having a 10 mm wide blade 626, or more or less,constructed from a linear piece of material which is attached to thewiper support 625. The wiper blade 626 may be comprised of materialscapable of removing debris, such as, such as rubber (e.g., buna,ethylene propylene diene monomer (EPDM), silicone, etc.) or plastic(urethane, polyethylene, etc.). The wiper blade 626 may also becomprised of a solid or sponge-like foam or fabric (e.g. woven fabric,felted fabric, etc.). In some embodiments a wiper may include multiplewiper blades.

The wipers may be attached to a motor. In this regard, a wiper arm maybe attached directly to a motor or to a motor via a bearing ring thatrotates around the base of the dome. For instance, as further shown inFIG. 6 , a first end of the wiper arm 635 may be attached to a first endof a shaft 645 and the opposite end of the shaft 646 may be connected toa bearing ring 650. The second end of the wiper arm 636 may be attachedto the wiper support 625. The bearing ring may be rotated by a motor660. The motor may rotate the bearing ring and accordingly the shaft 640and wiper arm 630 in the first direction 130 causing the wiper 601 toalso rotate around the sensor cover 115, as further shown in FIG. 6 .The wipers may be rotated in the same direction or a different directionthan the rotatable sensor 320 (not shown).

The rotation rate of the wiper rotating around the sensor cover may beset such that the wiper is positioned outside the field of view of acamera as the camera captures an image. In this regard, when only asingle wiper, such as wiper 601 is used, the wiper 601 may be rotated atsuch as speed to locate the wiper approximately halfway between theapertures of neighboring cameras, in a part of the dome with noaperture, when the camera is imaging. The rotation rate of the wiper maybe determined using the following Formula (1):wiper_advances_per_cycle/sensor_advances_per_cycle*f_sensor_rotation,where wiper_advances_per_cycle is the number of cameras separations(i.e., the spaces between the cameras,) a wiper will advance per cycle,f_sensor_rotation is the rotation rate of the rotatable sensor, andsensor_advances_per_cycle is defined by Formula (2):sensor_rotations_per_cycle*num_sensors+wiper_direction*wiper_advances_per_cycle,where num_sensors is the number of stationary sensors, wiper_directionis 1 when the wiper rotates in the same direction as the rotatablesensor and −1 when the wiper rotates in the opposite direction as therotatable sensor, and sensor_rotations_per_cycle is the number ofrotations the rotatable sensor rotates every cycle, where a cycle is thedifference between wiper position and rotatable sensor position repeatswith some periodicity. For instance, the rotatable sensor may rotate onefull rotation plus one slot while the wiper rotates one slot in a cycle.A slot is the area between stationary sensors.

In some instances, upon determining to wipe the sensor, the rotation ofthe wiper may be delayed until the location of the LIDAR is at astarting location relative to the wiper blades. In this regard, thewiper may transition from standstill to the desired wiping speed whileminimizing interference of any imaging cameras while the wiper rotationbegins. The starting location may be determined such that when the wiperbegins to move, but before reaching the desired rotation rate, the errorbetween the desired azimuth and their actual azimuth of the wiperrelative to the LIDAR and imaging cameras is minimized. For more thanone wiper, the starting location for each wiper relative to the LIDARand imaging cameras may be different.

FIGS. 7A-7D show an example of a single wiper rotating around the sensorcover such that it is positioned outside the field of view of thecameras as the cameras capture images. Turning first to FIG. 7A, LIDAR320 is initially directed towards camera 300 and wiper 601 is positionedon the sensor cover 115 such that it is in the slot between cameras 300and 314. Upon starting up, the LIDAR 320 and cameras may begin tocapture data immediately. In this regard, since LIDAR 320 is alignedwith camera 300, camera 300 may capture an image, as illustrated by thestar in FIG. 7A. For clarity, not all of the cameras are labeled inFIGS. 7A-7D. As described in Formula (1), the wiper may also beconfigured to rotate in the opposite direction of the rotatable sensor.

As the LIDAR 320 rotates, for instance at a speed of 10 Hz, in a firstdirection 710, the wiper 601 may begin to rotate in the direction 710 aswell. In this regard, since there are 8 cameras (i.e., stationarysensors) situated in the sensor and configured as described above, thewiper 601 may rotate at a speed of 1.11 Hz, in accordance with Formula(1). By rotating at a speed of 1.11 Hz, the wiper 301 is positionedbetween apertures of neighboring cameras when a camera is imaging. Forexample, FIGS. 7B-7D show the wiper 601 continuing to rotate in thefirst direction 710 as the LIDAR 320 rotates. Upon the LIDAR 320aligning with a camera, such as camera 302, in FIG. 7C the wiper may bepositioned in a slot between cameras 300 and 302, such that the wiper isoutside the field of view of camera 302 as it captures an image.

To provide more rapid clearing of the sensor cover the speed of therotation of the wiper may be increased. For instance, the rotation ofthe wiper may be increased to 2 Hz. However, as the speed of the wiperincreases to a certain point, the operation of the wiper may becompromised as centripetal forces may result in the contact force of thewiper on the sensor cover decreasing. Moreover, the longevity of thewiper may be decreased as the rotation speed is increased, since thewiper blade may wear out more quickly. As such, more than one wiper maybe used.

For instance, a second wiper 801 may be used in conjunction with thefirst wiper 601, as shown in FIGS. 8A and 8B. The motor 660 may rotatebearing ring 650, which in turn, may rotate wiper shafts 640 and 840 andwiper arms 630 and 830 of the first wiper 601 and wiper 801,respectively in a first direction 130. The wipers may therefore move inthe same direction at the same speed, thereby increasing the clearingspeed of the sensor cover 115 relative to a single wiper rotated at thesame speed.

However, when the second wiper 801 is positioned at 180 degrees apartfrom the first wiper 601, as shown in FIGS. 9A-9D, the second wiper 801may obstruct a camera's field of view during its exposure duration.Turning first to FIG. 9A, LIDAR 320 is initially directed towards camera300, wiper 601 is positioned on the sensor cover 115 such that it is inthe slot between cameras 300 and 314, and wiper 801 is positioned in theslot between cameras 306 and 308. Upon starting up, the LIDAR 320 andcameras may begin to capture data immediately. In this regard, sinceLIDAR 320 is aligned with camera 300, camera 300 may capture an image,as illustrated by the star in FIG. 9A.

As the LIDAR 320 rotates at a speed of 10 Hz, in a first direction 710,the wipers 601 and 801 may begin to rotate in the direction 710 as well.In this regard, since there are 8 cameras (i.e., stationary sensors)situated in the sensor and configured as described above, the wipers 601and 801 may rotate at a speed of 1.11 Hz, in accordance with Formula(1). By rotating at a speed of 1.11 Hz, the wiper 301 is positioned atcamera 300 when camera 308 is imaging. However, wiper 801 may bepositioned in front of camera 308 as the camera captures an image,thereby obstructing the camera's field of view, as shown in FIG. 9B. Asthe sensor continues operation, both wipers may be positioned outsidethe field of view at certain times, as illustrated by FIG. 9C, howeverwiper 801 may again obstruct the field of view of camera 310, as camera310 captures an image as shown in FIG. 9D.

The second wiper may be offset from the first wiper at a predeterminedangular distance relative to the center of the sensor cover to preventthe second wiper from being positioned in the field of view of astationary sensor when the stationary sensor is capturing an image. Inthis regard, the offset of the second wiper to the first wiper may bedetermined using the following Formula (3):((slot_numbers+wiper_direction/2)/num_sensors*2*Å*(1.0−wiper_direction*wiper_frequency/f_sensor_rotation)),where slot_numbers is an index of the position between cameras that agiven wiper should be in during the first rotation of the rotatablesensor. For instance, as further shown in FIG. 9 , there may be eightslots between the eight cameras. Formula (3) may be used for more thantwo wipers.

Using Formula (3), the two wipers 601 and 801 may be positioned 160degrees apart, as shown in FIGS. 10A-10D. Continuing the example of theLIDAR rotating at 10 Hz, each of the two wipers 601 and 801 may berotated in direction 710 at a rate of 1.11 Hz, as further shown in FIGS.10A-10D. Accordingly, each wiper is positioned outside the field of viewof any imaging cameras, such as cameras 300, 308, 302, and 310 in FIGS.10A-10D, respectively.

FIG. 11 shows a plot 1100 of two offset wipers on the sensor cover overa period of time relative to the position of the sensor cover at whichthe cameras are imaging. In this regard, the wipers may be rotated at aspeed of 1.11 Hz and offset 160 degrees apart. Lines 1101 and 1102represent the path of the wipers around a sensor cover over a period oftime. Lines 1110 represent the direction the rotatable sensor ispositioned over the period of time and marks 1112 represent locations onthe sensor cover where images are being captured by stationary sensorsat particular times. As can be seen from plot 1100, the two offsetwipers are never positioned at locations which would obstruct a camerawhere images are being captured by that camera. For clarity, not alllocations on the cover sensor cover where images are being captured arelabeled.

Similarly, FIG. 12 shows a plot 1200 of three wipers 1201, 1202, and1203 rotating at a speed of 1.11 Hz on a sensor cover over a period oftime relative to the position of the sensor cover at which the camerasare imaging. Lines 1210 represent the direction the rotatable sensor ispositioned over the period of time and marks 1211 represent locations onthe sensor cover where images are being captured by stationary sensorsat particular times. As can be seen from plot 1200, the three offsetwipers are never positioned at locations which would obstruct a camerawhere images are being captured by that camera. For clarity, not alllocations on the cover sensor cover where images are being captured arelabeled, nor are all lines representing the direction the rotatablesensor is positioned label.

Most of the foregoing alternative examples are not mutually exclusive,but may be implemented in various combinations to achieve uniqueadvantages. As these and other variations and combinations of thefeatures discussed above can be utilized without departing from thesubject matter defined by the claims, the foregoing description of theembodiments should be taken by way of illustration rather than by way oflimitation of the subject matter defined by the claims. As an example,the preceding operations do not have to be performed in the preciseorder described above. Rather, various steps can be handled in adifferent order, such as reversed, or simultaneously. Steps can also beomitted unless otherwise stated. In addition, the provision of theexamples described herein, as well as clauses phrased as “such as,”“including” and the like, should not be interpreted as limiting thesubject matter of the claims to the specific examples; rather, theexamples are intended to illustrate only one of many possibleembodiments. Further, the same reference numbers in different drawingscan identify the same or similar elements.

The invention claimed is:
 1. A method of clearing a sensor cover ofdebris with a first wiper, comprising: rotating the first wiper in afirst direction at a first predetermined rotation rate defined at leastin part by a second predetermined rotation rate of a first sensorarranged within the sensor cover.
 2. The method of claim 1, wherein thefirst predetermined rotation rate is further defined by a total numberof a plurality of second sensors, the second sensors being fixedrelative to the sensor cover.
 3. The method of claim 2, wherein thefirst predetermined rotation rate is further defined by a number ofspaces between a plurality of second sensors the first wiper willadvance per cycle where a cycle is the difference between the firstwiper position and a position of the first sensor repeats with someperiodicity.
 4. The method of claim 3, wherein the first predeterminedrotation rate is further defined by dividing the number of spacesbetween the plurality of second sensors by a number of spaces betweenthe plurality of second sensors the first sensor will advance per cycle.5. The method of claim 3, wherein the first predetermined rotation rateis the result of the dividing, multiplied by the second predeterminedrotation rate of the first sensor.
 6. The method of claim 2, wherein thefirst predetermined rotation rate is defined by a formula:wiper_advances_per_cycle/sensor_advances_per_cycle*f_sensor_rotation,where wiper_advances_per_cycle is the number of spaces between theplurality of second sensors the first wiper will advance per cycle,f_sensor_rotation is the second predetermined rotation rate, andsensor_advances_per_cycle is the number of spaces between the pluralityof second sensors the first sensor will advance per cycle where a cycleis the difference between the first wiper position and a position of thefirst sensor repeats with some periodicity.
 7. The method of claim 1,wherein first sensor rotates in a second direction at the secondpredetermined rotation rate.